Prepress Guidelines
Trapping

This guideline is intended to aid in preparing the type of trapping typically done for illustrations. It should not be considered a comprehensive class in computer prepress. Read your manuals for more complete information, and don't hesitate to take a class if one is available to you. Some full service printers actually hold periodic seminars, and others are very open to providing help to artists who have worked with them. It can be very helpful to create simplified and greatly enlarged examples of trapping problems and then run separations on these tests to see the results.

Basic Concepts

When creating art on the computer, it is important to remember that your work is directly used to make the film that makes the printing plates for the press. It is important to have a basic understanding of prepress and printing and to keep this knowledge in mind while constructing your files. This knowledge will allow you to create art that will output and print properly.

A trap is an area of overlap between 2 colors caused by printing one color over the other along an adjoining edge. You have probably heard the terms CHOKE and SPREAD. A CHOKE is a trap that brings an outside color within the outline of a shape. A SPREAD is a trap that extends the shape outward into the outside color.

When deciding whether to trap or overprint, you need to know which elements will be on different pieces of film when output. Do not confuse this film with overlays you may have cut in the past for traditional mechanicals. Different screens or patterns do not end up on different pieces of film. There will be one piece of film for each color being printed, to create one printing plate for each ink. Your traps are needed to ensure that if registration slips on the printing press, there is a slight overlap anywhere these colors touch to keep some color in the accidental gap that occurs. Without a trap, such a gap would be a very obvious and glaring white.

When you check OVERPRINT in the Paint Style Box, it means that this item will print over top of anything positioned below it. When you do NOT check OVERPRINT, it means that this item will drop out of anything positioned below it.

When an object is filled and stroked, the fill always extends to halfway through the width of the stroke. The stroke is always on top of the fill.

When you create a TRAP, the overlapping area of the two colors should be 0.25 pt. Only if necessary, in a tight space, you may go as low as 0.175 pt. but never any lower. Remember, if your trap is created by a stroke, ONLY THE OVERLAPPING AREA IS THE TRAP (0.25 or 0.175 pt.).

2-Color Line Art

100% Black Placed Over Color need only be Overprinted. 100% Black is opaque enough to cover whatever is under it. Also, remember that black is always the last ink to be printed. The standard leader line, labels, and any color area with a black outline would fall into this category.

Screened Black Placed Over Color is Overprinted when you want to combine the colors. If you want the screened black to appear gray, it must be Trapped. This means having two layers of the screened black above the color. One layer is larger by the trapping measure and Overprints, the other layer is smaller and does NOT Overprint. The result is an area of pure gray bounded by an area of combined black and color. Strokes around fills can also be used.

Color (Any %) Placed Over Black (Any %)–Note that this situation calls for construction techniques similar to the last scenario, but reversed. Overprint when you want to combine the colors. If you want the color to appear pure, it must be Trapped. This means having two layers of color above the black. One layer is larger by the trapping measure and Overprints, whereas the other layer is smaller and does NOT Overprint. The result is an area of pure color bounded by an area of combined black and color. Strokes around fills can also be used.

Color (Any %) Placed Next to a Screen of Black–2 colors that abut each other must be Trapped. The precise method can vary, but an area of overlap must be constructed between them.

If your shape has a stroke around it, this stroke may be used for the Overprint if it is a suitable size. Note that in the last two examples, where the shapes OVERPRINT the trapping rule, that rule could be made lighter than either color area to avoid an obviously dark overlap. Just remember that it must be strong enough to optically obscure any registration gap that might occur.

4-Color Line Art

4-color art can often be easier to trap than 2-color art. If you always keep in mind the 4 plates that are your end-product, you will soon notice that, frequently, adjoining color areas will share enough color on one plate to provide a suitable trap. For example, an orange (50% M/40% Y) abuts a purple (50% C/40% M). When this image separates, the magenta plate will have a continuous flow of color between the two shapes. At one point the % of magenta will change from 50 to 40, but whichever way the registration shifts, there will be magenta in the gap.

Keep in mind the strength of the colors utilized, however, since 5% yellow in a registration gap between two strong colors could be almost as glaring as white.

When no suitably overlapping color exists, a Trap must be constructed. Techniques are essentially the same as those discussed in the section on 2-color art, but keep in mind that you must choose which of the 4 colors would create the best trap, depending on the screen values and position of the colors as they are used in that piece of art. Note that, as above, a trapping rule placed below overprinting art may be lighter than its "parent" color or even contain a lighter combination of colors. This can put more color into a gap, while putting less "foreign" color into each side.

It is important to pay attention to the black. When 100% black is used for outlines around color, leaders, or type over color, it will need to be Overprinted. Remember that black prints last on the press and will cover anything under it.

~ Prepress Guidelines ~ Multimedia Guidelines ~
~ Introduction ~